[ES] Conventional Mooring Reaches Deeper

There is a huge demand for high-spec drilling rigs, and in remote areas of the world where comparatively little deepwater drilling is taking place, extending a rig’s mooring capability is one avenue operators have taken to contend with asset constraint.

Hart's E&P, October 2008

Author: Judy Murray

Solstad’s Normand Ivan was one of the AHVs that helped to carry out the record-setting mooring offshore Malaysia. (Image courtesy of Solstad ASA)

The shortage in rigs capable of drilling in deep water has restricted exploration activity, particularly in places where drilling activity is limited. Every drilling rig is constructed with design specifications that dictate where and under what conditions it can work, but as InterMoor proved in late June 2008, those specifications are not absolute.

Setting the stage
The Ocean Rover is a fifth-generation conventionally moored Victory Class semisubmersible originally rated for operation in 1,500 ft (457 m) water depth. That depth limit was upgraded in 2003 to 6,500 ft (1,981 m) at the Keppel FELS yard in Singapore, with the proviso that the rig could operate at greater depths on a case-by-case basis.

According to Dale Bradford, Malaysia drilling manager for Murphy Oil Corp., the opportunity to test the limits of the Ocean Rover’s capability surfaced with the company’s need to drill a deepwater prospect offshore Sabah, Malaysia, that exceeded the rig’s depth capability.

“InterMoor holds a long-term contract for these kinds of services with Murphy Malaysia,” Bradford said. “That contract was awarded initially because of the experience and quality of people they can bring to the table.”

According to Derrick Laskowski, an InterMoor engineer who worked on this project, extensive computer modeling preceded the decision to attempt this extended conventional mooring project which, when executed, set a world record.

Engineers at the InterMoor office in Houston used bathymetry maps for the Buntal area provided by Murphy, Laskowski explained. “Mooring analysis begins with a bathymetry drawing of the field that shows infrastructure that would limit anchor placement. When the environment has been established, we come up with horizontal distances and mooring patterns and plot a design. Next, we determine how much wire and chain, anchor components, and anchor holding capacity are required for this environment. We use a program called GMOOR, a dynamic analysis program used to simulate the response of the rig and mooring system to any input environment data (wind, wave, and current headings) at 5º increments over 360º to identify how the rig will perform.” Part of the mooring analysis employed a program called QMOOR, a single-line catenary analysis tool that allowed engineers to model deployment and installation operations.

With the basic mooring pattern plotted and tested, the next step was to assess the rig’s stability with one of its mooring lines broken. API requires this analysis to be run for each of the mooring lines, which in the case of the Ocean Rover meant that the analysis was carried out eight times using a 10-year return period, Laskowski said.

The modeling was done in two phases. “The first phase started over a year before the well was spudded,” Bradford said. “The reason for that was Murphy’s concern about the drilling rig’s capability to be anchored in that kind of water depth without a major upgrade. We had to be sure the rig could handle the project. It was important to determine that the boats could handle the work as well.”

The sophisticated model produced by InterMoor was critical in convincing Murphy and Norway’s Solstad, the company that supplied the anchor-handling vessels (AHVs), that the work could be done, Bradford said.

Making it happen
Dewey Fleetwood, operations manager for Southeast Asia for InterMoor, was tasked with making sure the project could be safely executed with the equipment and personnel available.

“When we start a job, it begins with the Houston engineering department performing the mooring analysis,” Fleetwood explained. “When the analysis is approved, I write the procedures and return them to Houston for review and approval. Those documents are the starting point for the actual job.”

On the rig, Fleetwood’s job was to coordinate the efforts of the captain of the Ocean Rover, offshore installation manager, and the barge captain. “I’ve been working with the guys on this rig for five years,” Fleetwood said, “and I’ve been working with Murphy ever since they started their projects here in Malaysia. I have a very good working relationship with the Diamond Offshore staff and with Murphy Oil. We call it ‘Team Rover.’ Everyone is out to help everyone else.”

Putting it all together
In the course of this mooring operation, the semi was moored at depths that required two of the anchors to be set in more than 8,000 ft (2,438 m) of water. The deepest leg of the eight-leg conventional mooring system reached 8,430 ft (2,570 m).

One of the prime factors in being able to carry out this job was load sharing, Fleetwood said. “We have a load-sharing chart that we stick to so all of the vessels and the rig stay well within their limits and can work safely. This minimizes the loads and distributes weight safely as the installation moves ahead.”

The choice of AHVs was critical to this operation. According to Bradford, the vessels were chosen for the experience and skill of the crews and for the vessel specifications. “These are high-spec vessels,” he said, “but they were not designed to work in these kinds of water depths. We had to take into consideration how far we were going to push their limits. InterMoor’s sophisticated model was key in convincing us that the work could be done.”

Careful preparation preceded the work, but the proof lay in successful implementation. “The real challenge is when you get into the field,” Fleetwood said. “A lot of times in the field, things don’t go as anticipated. You have to make adjustments as you go. It’s important to keep an eye on the rig and the AHVs, to determine quickly if the load is too high, and to know how to compensate to fix the situation. Everything has to be done safely.”

In this case, each mooring leg, deployed using the Normand Ivan and Normand Atlantic AHVs, consisted of a 10-metric-ton Stevpris anchor with the rig’s self-contained wire and chain. All of the anchors, including the deepest at 8,430 ft (2,570 m), were set without incident.

“It took the two vessels five to six hours to install each of the eight legs,” Laskowski said. “From the time we started until we finished, the operation took about two days.”

Bradford attributes some of the success of the operation to the shore-based logistics people and supervisors who made sure everything worked smoothly when boats arrived to pick up materials at the yard in Labuan.

With the job completed, the semi has moved offsite, increasing confidence in this approach. “Forty-some days later, when the anchors were recovered, the tensions we encountered were well within the predicted limits. Now we know what we can expect in terms of mooring operations and in terms of breakout tensions in deepwater,”

Fleetwood said. “There is a lot of deepwater acreage offshore Malaysia. With this conventional mooring achievement, we have established that rigs like the Ocean Rover can be safely moored there.”

Bradford agreed. “The value of this mooring record for Murphy is that the company is using a rig that was not rated for this water depth to drill in a frontier deepwater area,” he said. “This job gives Murphy the confidence to drill more of its deepwater acreage offshore Malaysia with its current rig capability.”